Hello, so you are only able to print to the machine if the stacker is off correct? Might be something wrong with the stacker if it is going straight to the error code. Does the stacker actuate or anything when you power on the machine? You also stated you had another similar model, I would try adding the stacker to that machine and see if it does the same thing, this way at least you can rule out the printer itself.
Hope this helps.....